It is necessary to reduce, to the least possible extent, hard nonmetallic inclusions contained in steels for forming very fine steel wires whose diameters between 0.1 and 0.5 mm made by a cold wire drawing process and springs required to have high fatigue strength. The nonmetallic inclusions cause the breakage of steel wires during wire drawing and reduction of fatigue strength. Therefore, high-cleanliness steels containing the least unavoidable nonmetallic inclusions are used for the foregoing purposes.
Demand for weight reduction and output enhancement in automobiles has been increased in recent years to reduce exhaust gases and to reduce fuel cost. Therefore, high-stress designing of valve springs included in engines, and suspension springs included in suspension systems is a recent trend of design. Thus there is a tendency to increase the strength of spring steels and to decrease the diameters of springs. Consequently, load stress becomes higher. Thus, there is a demand for high-performance steels having even more excellent characteristics resistant to fatigue and sag. Valve springs are required, in particular, to have the highest fatigue strength.
The strength of very fine steel wires represented by those forming tire cords has been progressively increased to reduce the weight of tires. Recent steel cords have a strength on the order of 4000 MPa. Since very fine steel wires having higher strength are more easily breakable during cold working (wire drawing), the improvement of cold workability of such steel wires having high strength is desired.
As mentioned above, steel springs and very fine steel wires of high-strength steels are more subject to fatigue fracture or breakage due to nonmetallic inclusions contained in the steels. Thus the severity of demand for the reduction of nonmetallic inclusion content and size has been progressively increased.
A variety of techniques have been proposed for the reduction of hard nonmetallic inclusion content and size. Results of studies of preventing fatigue fracture are introduced in, for example, “126th and 127th Nishiyama Memorial Technical Lecture”, The Iron and Steel Institute of Japan, pp. 145-165, Nov. 14, 1988 (Reference 1). According to the results of studies mentioned in Reference 1, the fatigue fracture of spring steels does not occur when inclusions contained in the spring steels are those of a CaO—Al2O3—SiO2 system having a melting point between 1400 and 1500° C., and the reduction of nonductile inclusions, such as Al2O3, contained in tire cords is effective in preventing fatigue fracture. Means for making inclusions not detrimental are proposed in JP-B 6-74484 (Reference 2) and JP-B 6-74485 (Reference 3). Means mentioned in Reference 2 shows that inclusions are fractured and dispersed and become not detrimental during cold working or wire drawing when the composition of inclusions is 20 to 60% SiO2, 10 to 80% MnO, 50% or below CaO and 15% or below MgO. Means mentioned in Reference 3 shows that inclusions are fractured and dispersed and become not detrimental during cold working or wire drawing when the composition of inclusions is 35 to 75% SiO2, 30% or below Al2O3, 50% or below CaO and 25% or below MgO. However, further improvement of the properties of steels is necessary to meet recent quality requirement.
A technique proposed in JP-A 1-319623 (Reference 4) makes a high-cleanliness steel by mixing a mixture of a deoxidizer of a Si system and an alkaline metal compound in a molten steel to control the composition of the product of deoxidation such that the product contains the alkaline metal. The alkaline metal decreases the melting point of hard nonmetallic inclusions of Al2O3 and SiO2 systems. The hard nonmetallic inclusions having a low melting point can be extended in filaments during hot rolling and become not detrimental to the drawability and fatigue characteristics of the steel. Possible alkaline metals are Na and Li, which have the same effect. The alkaline metal singly added to the molten steel adversely affects the yield and hence it is recommended to use the alkaline metal together with the deoxidizer. For example, LiF is added together with sodium silicate to a part, in which stirring Ar bubbles appear, of the molten steel poured from a converter into a ladle at an initial stage of a ladle furnace process (LF process).
A technique proposed in JP-A 2-15111 (Reference 5) adds an alkaline metal to a molten steel to decrease the melting point of inclusions and to change the shape of inclusions during hot rolling. Possible alkaline metals are Li, Na and K, which have the same effect. Since the alkaline metal does not dissolve in the molten steel, it is recommended to use Si for dilution. More concretely, a Si alloy containing 12% or below Li is used as a deoxidizer.
A technique proposed in JP-A 2002-167647 (Reference 6) adds an alkaline metal oxide to the molten steel to improve the ductility of inclusions including SiO2 as a principal inclusion. According to Reference 6, the improvement of the ductility of the inclusions is achieved by reducing the energy of interface between the inclusions and the molten iron by the alkaline metal instead of by decreasing the melting point as mentioned in References 3 and 4. In all cases, the alkaline metals Na, K and Li are thought to be equivalent. The alkaline metal content of slag is on the order of 10% at a maximum. Practically, only Na is used.
A technique proposed in JP-A 2002-194497 (Reference 7) recommends using an alkaline metal oxide for Si-deoxidation. This technique uses an alkaline metal oxide because the alkaline metal oxide is able to reduce effectively the activity of SiO2 contained in ladle slag and, consequently, the oxygen content of the molten steel can be reduced. Possible alkaline oxides recommended by Reference 7 are Na2O, K2O and Li2O, which have the same effect. The technique proposed in Reference 7 differs from the technique proposed in Reference 5 in adding Li to the molten steel. More concretely, Li2O in the form of a carbonate is mixed in slag and the Li content of the slag is on the order of 8% at a maximum.